Ernst abbe



E. ABBE. DOUBLE PRISM FOR TOTALLY RBFLEGTING RBIRAGTUMBTBRS.

Ptented Oct. 22, 1895.

staffa UNITED STATES PATENT OFFICE.

ERNST ABBE, OF JENA, GERMANY, ASSIGNOR TO TIIE FIRM F CARL ZEISS,

0F SAME PLACE. i l Y DOUBLEPRISM FOR TOTALLY-llEFLECTING REFRACTOMETERS.

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SPECIFICATION forming pm ef Letter; Patent Ivo. 548,495, dated oetabcr 22, Issa.

appunti and october ze, 189s. sum m4119327. csa man Piuma n om, Jugar; s, 1892.1. 65.303 1 h ram .nm ao, 1m, n. 222,728, mi in Engine www 1o, 1892, x. 22.751.

To all whom it may concerm' Be it known that l, ERNST ABBE, doctor of philosophy, a subject of the Grand Duke of SaxeWeimer, residing at Jena, in the Grand 5 Duchy of Saxe-Weimer-Eisenach, Germany, have invented new and useful Improvements in Double Prisms for Totally-Reflecting Refraetometers, (for which I have obtained patents in Germany No. 65,803, bearing date Janxo nary 5, 1892; in France No. 222,728, bearing date June 30,1892, and in England No.22,5l, bearing date December 10, 1892,) of which the following is a specification.

This invention relates to double prisms to x 5 be employed in the construction of totally-refleeting refractometers, and has for its object to enable the prism itself to produce the achromatization of the critical line of the total rb Ilection.

zo It is known that the differences which exist between the refractive power of liquids or semiliquids can bo accurately ascertained and measured by inclosing the substance to be observed in the form of a thin layer or film bez5 tween two glass prisms, the angles of which are equal but opposite, and determining the angle of incidence under which the light has to be caused to pass through these two prisms to undergo total reflection upon the interme- 3o diate layer of liquid. This method has a1- ready been applied to practical purposes-- such, for example, as the determination of the degree of purity of oils and fats, the degree of concentration of solntions,or the like. Let it be supposed that the liquid constitoting the aforesaid intermediate layer possesses a refraction inferior to that of the glass` If under these conditions an illuminated surface is looked at by the medium of a double 4o prism contniuiug'the said liquid in the form of a thin layer between its two constituent prisms, one halt of the field of view appears` light, while the other half is perfectly darli, owing to the total reflection of the luminous rays. lly the aid of a telescope it is possible to observe, either upon a scale arranged within the field of the eyepiece or upon a graduated arc of the circle, on the center of which the telescope turns, the direction in which the 5o critical line of the total reflection is placed relatively to the faces of the prism, and nally to deduct therefrom the refractive index of the liquid constituting the intermediate layer. The apparatnig which have heretofore been based upon this method of observation in view of practical purposes, and which are termed totallyreflecting refractometers, present the defect that the critical line of total reflection is not achromatic. This is due to the fact that the position of the said line is dif- 6o ferent for the diiIerently-eolored constituents of white light. In order to obtain 4au accurate observation of this critical line, it is necessary to have recourse to homogeneous or monochromatic light, or in the case of white light to complicate the apparatus by providing it with a special arrangement (compensator) acting so as to compensate for the dis persion of colors occurring when the reticetion is total by a prismatic dispersion.

The improvements forming the object of the present invention enable these diiliculties to be overcome in refractometers employed for verifying or examining a definite substance or adetiuite class of substances-such, for example, as a definite kind or class of oil or fat, a detinite kind of solution, or the like. In this case the achromatization of the critical line of total reflection can he obtained by the double prism itself, and this with adegree of 8o precision su flicient for all practical purposes. Por this purpose it is simply necessary to construct the double prism in such a manner that when the reflection is total the dispersion taking place at that moment on the limit be tween the glass and the liquid will be nentralized by an equal but opposed dispersion on the face of emergence of the prism on the side of the observer. It the optical constants, refractive index, and dispersive power or dis- 9o persion of the liquid to be tested, as well as ot the glass the double prism is to be constructed with, are known, any opticiau versed in the treatment of problems of this kind will be enabled by the followingdireclions to numcrically determine according to known methods the elements required for the prism. Ou the annexed diagram,1 and P'nre the two prisms. l. ls the liquid interposed betweenthesame;l,theliinitingsurfacebctween roe i angle of refraction of the prism, and i the angle of incidence relatively to the face of emergence of the prism.

The optical signs employed in the following denote: nrmean index of refraction of the ro liquid. nbzindex of refraction of the liquid relatively to the D iine of the spectrum. ne:

index of refraction ofthe liquid relativelyto Prannhofers line C. nr'T-iudex of refraction of the liquid relatively to Fraunhofers line t5 F. A n :difference between the refractive indices of the liquid for two different colors. =dispetsive power or dispersion of the liquid. N=mean index of refraction of the 2 glass. L X=difference between the refractive indices of the glass for two different colors. -L-qrdispersive power or dispersion of the glass.

Supposing it be known the refractive index n and thedispersive power -An-n of the liquid or body to be tested, as well as the-refractive index N and the dispersive powerNI of the glass to be employed in constructing the prism, by using well-known methods of calculation-viz., by means of the formnlzc given at the end of this paragraph-we first determine the critical angle e of the total reflection for the mean color, and then the difference between the critical angles e of the total reflection for two different colors-as, for instance, Fraunhofers lines C and F. This difference constitutes the measure for the disperston on the limit surface I, occurring with the said liquid and glass on the reflection being total. Then we proceed to determine the refractive angle required for the prism, and finally that angle of incidence i on the surface of emergence E with which the dispersion produced by the refraction of the light on thissurface will beof equal magnitude, but of reverse character, as the dispersion that takes place on the surface l when the rellec tion becomes total. The several formulae to be used for these calculations are,iu theiror.- der of application,

First. N sin. e=n.

Second. Tang. (i-e):

'usual manner, or the double prism may be Heftes Thcaforesaid condition for the construction I of the prism can be realized with any kind of glass within certain limits. In order, how- 7o ever, to reduce as mttch as possible the anglo i at which the light is to be looked at through vthe prism, the latter should be composed of n I l glass of which the quotient in@ approaches ne A nearly as possible the quotient l-i of the liquid or body to be tested.

It is most advantageous to employ ln the f composition of the prism a. kind of glass with which the value of the aforesaid quotient up: t preaches as nearly as possible that of the su i stance in question. 1n this case the necesz sary compensation can be obtained with. small angle of incidence i or even with pera 3S pendicular incidence. 3 The following are given as examples of the g construction of the double prisms for refrac p 1 tometers, according to this invention.

EXAMPLE L-Double prism to be employed 9 in the examination of butter.

Butter, melted 1.4572 ovm glass 0.83 1.5395

in [he examination ofsolution. offal ofm in ether. t

Solution of fat of milk ot .venge t strength Lm lm Crown 11:50.83 1.4907 0.411765 0.0m

In this case the values of the angles are as U5 follows: 6:66 3G'. 9:58" 10'. `=12 42'.

The double prisms constructed as hereinbefore described may be made of any required dimensions. They may be provided with any l kind of supports or cases according to the purposes for which they are required, such as cases made with hollow sides or walls for the purpose of regulating the temperature by means of hot water, for example. A telescope is combined with the prism in the usual man ner. In order to measure differences in the refractive power of the substances under examination, a graduated scale may be coinbincd with the lens of tho eyepiece in the 13o rotatably arranged relatively to the axis of the tclescopeand combined with a graduated nrc of a circle. When the substances to be examined possess the sume refractive power,

but d'erentiate with regard to their dispersive power, it is sufficient to observe the colored critical line in place of the aclaromatized line, and in this case the assistance of means for reading olf the results and adjustug the apparatus and the telescope may all be dispensed with.

In totally reilecting refractometers for testing definite substances or classes of substances, a double prism so constructed that the refraction angle y of the prism and the angle of incidence i on the face of emergence of s':\me,canse a dispersion of 'the light on the said face equal in amplitude but of opposite l5 character to the dispersion occurring on the limit face l when the reflection is totaL In testimony whereof I have hereunto set my hand in the presence of two subscribing 

